The purpose of the present investigation is to improve our current understanding of the control of the pulmonary circulation by humoral, ionic and neurogenic factors. The effects of prostaglandins, endoperoxides, thromboxanes and prostacyclins will be investigated on the pulmonary and systemic vascular beds in several laboratory species. New techniques for evaluating the effects of humoral factors on the pulmonary vascular bed under conditions of controlled pulmonary blood flow in the intact spontaneously breathing cat and monkey have been developed. The relationships between the effects of the prostanoids on the pulmonary vascular bed and on the airways will be assessed in the intact animal. The effects of thromboxanes and prostacyclins on intravascular aggregation of blood platelets will be assessed in the pulmonary vascular bed in the intact animal. In pilot studies, prostacyclin analogs were found to be potent pulmonary vasodilators in the feline pulmonary vascular bed in the intact spontaneously breathing cat. The prostacyclins were also found to be potent vasodilators in the feline mesenteric vascular bed and in the canine kidney. These studies suggest that the prostacyclins are among the most powerful naturally occurring vasodilator hormones in the body.